Get single line from QFile

Creatorczyk

Hi,

I would like to read a single line at position 5 from the QFile object and write it to QString. How could I do that?

jsulm

@Creatorczyk https://doc.qt.io/qt-5/qfiledevice.html#seek
https://doc.qt.io/qt-5/qiodevice.html#readLine-1

Creatorczyk

@jsulm I wrote this code to get 2 line from below file

file.txt

abcdef
ghijkl
mnoprs
tuvwxy

My code:

file.seek(2);
QByteArray line = file.readLine();
qDebug()<< QString(line);

But my output is:

cdef

What could I do to get below line?

mnoprs

J.Hilk

@Creatorczyk question, do you know, the exact length of your each line ? And do you know the end of line format ?

If the answer is yes, then you simply used seek false.

seek about = bytes in line * 2 to get to the start of line 3

if you don't know it, or it may vary, you'll have to use
https://doc.qt.io/qt-5/qiodevice.html#readLine-1

and count how often you read a line, discard everything before the count of 3

jsulm

@Creatorczyk seek() works with bytes, not lines. In your first post you did not say that "position 5" means line 5 and not byte 5. If you want to get the line number n you have to read and skip lines 0..n-1:

for (int i = 0; i < 2; ++i)
    file.readLine();
QString line = file.readLine();

Keep in mind: this code does not have any error handling (you could have less lines in the file than you expect).